It is claimed that a MMORPG is fundamentally different from a single-player RPG in that the MMO game has a persistent world: When you log off, the world continues to exist and things happen while you are away. In a single-player game the world freezes when you save it, and starts exactly there when you load that game. But is that really true? When you log back into a MMORPG, has the world really changed in your absence? Or isn't the change limited to other players having gained a level while you were gone, and the content of the auction house having changed?
In the current common model of eternal respawns, where it doesn't matter if somebody else killed the monsters you need for your quests while you were gone, because they came right back 5 minutes later, can we really talk of a persistent world? Or is a stupid Facebook game like Farmville, where somebody fertilized your crops while you were away, ultimately more persistent than a classic MMORPG?